The walk the pair, or group, if you include the dog, take in Lorna Crozier's "Crossing Willow Bridge," proves to be more than just a simple walk.
The mother, "seventy-six/this year," has recently become "unsteady/on her feet." The dog, a black lab, "All energy and muscle/and too much love," bumps the pair's legs and smacks the mother's legs with the stick. He makes the walk difficult.
Instead of a short, easy, leisurely walk, then, the walk becomes a trek, a journey, difficult and risky. The mother, being unsteady, could easily fall on her own. But the dog makes the possibility that much more likely.
The speaker also sees the dog as having "more to do/with time, how it runs ahead and keeps returning"--the dog, literally, and time, figuratively. Plus, the pair walks over the bridge, then back again.
The difficulty, the running of time, going back and forth over the bridge, all combine to make this walk more than a walk--they make it a journey.